But Justice Rosemary Nation has still found Allan Shyback guilty of the lesser offence of manslaughter rather than second-degree murder.
The judge says she was left with reasonable doubt about Shyback's intentions when he killed Lisa Mitchell five years ago.
Shyback testified he had been the victim of domestic abuse for nearly a decade and Mitchell had attacked him with a knife the day she was killed.
More than necessary force
Nation says the force Shyback used to fend off Mitchell was reasonable until the couple fell to the ground, but at that point it became more than necessary.
Mitchell was last seen in Calgary in October 2012.
An undercover police operation was launched in 2013 and ended with Shyback's confession and arrest in Winnipeg a year later.
Shyback has also been found guilty of causing an indignity to a body for putting Mitchell's remains in a Rubbermaid container and cementing it into a wall in the basement.